This invention was made during the course of, or under, a contract with the United States Department of Energy.
This invention relates generally to the art of displacement transducers and more specifically to displacement transducer employing interferometric apparatus in combination therewith.
In the art, displacement transducers, especially for use with inspection machines for gauging surface contours of precision machined parts, it has generally been the practice to use linear varible differential transformer transducers (LVDTs). LVDTs are essentially null devices, and their high accuracy (in order of 0.000010-inches) deteriorates rapidly when they move from the null position. This situation develops when the part figure differs considerably from the sweep movement capability of the inspection machine. Therefore, it is desirable to have an inspection transducer with comparable accuracy to present LVDTs but extended range and linear response. Also, it is desirable to have any new transducer be of the same physical size of current LVDTs and be connected to support instrumentation by simple cabling.
Earlier attempts to combine an interferometric system with a displacement gauge became very complicated and difficult to operate and maintain a degree of accuracy capable of an interferometer system, due to the critical alignment of the necessary optical components to direct a laser beam into the measuring optics carried by the probe stem of a displacement gauge. This is especially true in the sweep gauge in which the displacement gauge is mounted on a sweep arm of an inspection machine to monitor the contour of a machined part. The accuracy of this gauging system was destroyed because of the unwieldiness of the sweep arm necessary to carry the laser optic system and maintain proper alignment of the optics to direct the laser beam into the displacement transducer. For example, as the arm was moved from the vertical position in a sweep toward a horizontal position the gauging force, which must be maintained constant in order to make very accurate inspection of machined parts would vary due to the vector shift in weight of the arm carrying the displacement gauge and the interferometer optics. This problem alone creates errors which are greater than the range of error in which the inspection was to be carried out.
Thus, there is a need to provide a displacement transducer or similar device capable of producing a linear signal over a wide range of displacement measurements which may be readily adapted to use as a conventional LVDT transducer in an inspection machine.